1. Field of the Invention
The present invention relates to servo tracks written on magnetic tape to assist tape access machines in locating and positioning tape access heads on the magnetic tape and to otherwise access the magnetic tape.
2. Background Art
Magnetic tape is commonly used to store voice and data information due to its reliability, cost efficiency, and ease of use. Magnetic tape may be made more useful and cost-effective by increasing the areal density of information stored on the magnetic tape. This has generally been accomplished by including more data tracks on a given width of tape. While allowing more data to be stored, the increase in density of data tracks requires a narrowing of the width of the data tracks, a narrowing of the spacing between data tracks, or both. As the data tracks are made narrower or are more closely spaced, positioning of the tape with respect to the tape head becomes more critical to reduce the possibility of errors introduced while reading or writing.
Tape heads generally include read elements for reading data from the magnetic tape and write elements for writing to the magnetic tape. Typically, read elements may be formed in a read module with one read element for each data track that is to be simultaneously read. Similarly, write elements are manufactured into a write module, with one write element for each data track to be simultaneously written. Thin film construction techniques are used to achieve the small geometries required to produce read elements and write elements capable of accessing densely packed data tracks. To permit read-after-write operation on tape moving in either direction over the tape head, a typical tape head may include a sandwich of one write module between two read modules.
In order to increase the accuracy of positioning the tape head relative to the tape, servo tracks or stripes may be used to provide one or more reference points. One or more servo tracks may be used depending upon the number of data tracks which are placed upon the tape, the number of tracks simultaneously accessed, and the like. Servo read elements in the read modules or write modules sense tracking patterns on the servo stripe and produce signals which are received by a control system. The control system positions the head based on the servo signals.
One type of servo pattern allowing the position of a servo read element across the width of a servo track uses two sets of low frequency transitions in each servo frame. The two sets of low frequency transitions are recorded at a relative angle to each other at a given transverse location across the servo track. Thus, a time difference between accessing transitions in the first set and accessing transitions in the second set provides an indication of the servo read element location across the width of the servo track.
A servo track may contain information in addition to fine transverse location. For example, a servo stripe number may be encoded in the servo track for coarse transverse location. A longitudinal value may be encoded in some or all servo frames to indicate position of the access head along the tape length. One method for encoding such additional information is to vary the spacing between one or more low frequency transitions in each set of transitions. For example, the second transition in each set may be moved closer to the first transition to indicate a binary one and may be spaced equally between the first and third transitions to indicate a binary zero.
There are several problems associated with varying the spacing between low frequency transitions in one or more servo frames. First, the rate of information transfer is low, with typically only one bit communicated per servo frame. Second, the technique is asymmetric, requiring complicated logic in the tape access system to correctly interpret transition spacings when reading the tape in either direction. Third, positional shifting of low frequency transitions causes peak shifting of waveforms received from the servo read element, thus changing the servo read waveforms. The change in waveform shape requires additional electronics for correct interpretation. Fourth, the low frequency pattern is typically written by a single current driver, and thus cannot contain any information that varies between the servo tracks, such as a servo stripe number.
What is needed is to provide additional information in servo tracks containing fine positioning low frequency transitions that does not require modifying the low frequency transitions to convey this information.
The present invention combines low frequency transitions for fine transverse positioning with high frequency fields providing additional information.
A servo pattern recorded longitudinally along a length of magnetic tape is provided. The servo pattern has a plurality of servo frames, each servo frame having a first field of recorded low frequency transitions, a second field of recorded low frequency transitions, and at least one high frequency field recorded on the tape. The first low frequency field transitions are recorded on the tape such that a peak of each first field transitions varies in longitudinal position across the width of the servo frame. The second field of low frequency transitions are recorded on the tape such that no peak of any second field transition is parallel with the peak of any first field transition. At least one high frequency field defines a timing pattern. The timing pattern may be used to provide a reference for generating a clock signal that varies with tape speed.
In an embodiment of the present invention, at least one high frequency field defines servo data. This servo data may encode longitudinal position along the length of the tape, transverse position across the width of the tape, tape identification, and the like. The high frequency field defining a timing pattern may be two high frequency fields, one on either side of the high frequency field defining servo data.
In another embodiment of the present invention, each servo frame is symmetric with regards to longitudinal movement along the tape.
In yet another embodiment of the present invention, at least a subset of the servo frames provides the same information when read in one tape direction as information provided when read in the opposite tape direction.
A magnetic tape is also provided. The tape has longitudinal data tracks for storing data. The tape also has a plurality of servo tracks written along a length of the tape. Each servo track includes a plurality of servo frames. Each servo frame has a first field of recorded low frequency transitions and a second field of recorded low frequency transitions. The first field transitions are recorded such that a peak of each first field transition varies in longitudinal position across a width of the servo frame. The second field transitions are recorded on the tape such that a peak of each second field transition is not parallel with a corresponding peak of any first field transition. A high frequency servo data field is recorded in at least a subset of the servo frames. The high frequency servo data field indicates transverse position across the width of the tape.
A method of positioning a tape access head across the width of a tape is also provided. Transitions in a first low frequency field recorded on at least one of a plurality of servo tracks are sensed. Transitions in a second low frequency field recorded on these same servo tracks not parallel to the first transitions are also sensed. Fine positioning across the tape width is determined based on a time difference between sensing transitions in the first low frequency field and sensing transitions in the second low frequency field. A high frequency field recorded on the servo tracks is further sensed. Gross positioning across the tape width is determined based on the sensed high frequency fields.
A method of determining the position of a tape access head across the width of a tape is also provided. Transitions in a first low frequency field are sensed. Transitions in a second low frequency field oriented in a different direction than the transitions in the first low frequency field are also sensed. A high frequency timing field recorded on at least one servo track is also sensed.
In an embodiment of the present invention, a tape travel parameter is determined based on the high frequency timing field. Positioning across the tape width is determined based on the tape travel parameter and on a time difference between sensing transitions in the first low frequency field and sensing transitions in the second low frequency field.
A tape deck for accessing data tape is also provided. The tape deck includes a tape access head with servo read elements. Each servo read element operates to read one of a plurality of servo tracks written along a length of the tape. A servo mechanism positions the tape access head across the width of the tape. Signal conditioning electronics receive a servo signal from the servo read elements. Position determination logic receives the conditioned servo signal. The position determination logic determines the transverse position of at least one servo read element across the width of the servo track based on two sets of low frequency transitions recorded on the servo track. Timing extraction logic receiving conditioned servo signals extracts a clock signal from high frequency transitions recorded on the servo track as the tape moves past the tape head.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.